The present invention generally relates to bearings and, more particularly, to foil thrust bearings.
The ready availability of ambient atmosphere as a bearing fluid makes fluid bearings particularly attractive for high speed rotating machinery. Some applications might include, for example, a turboalternator-generator and turbocompressor.
Fluid bearings generally comprise two relatively movable elements (i.e., a bearing and a runner). A predetermined spacing between the bearing and runner is filled with a fluid such as air. Foils (or thin sheets of a compliant material) disposed in the spacing are deflected by the hydrodynamic film forces between the adjacent bearing surfaces. The foils thus enhance the hydrodynamic characteristics of the fluid bearing and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect.
To properly position the foils between the movable bearing elements, it has been common to mount a plurality of individually spaced foils on a foil bearing disk and position the disk on one of the bearing elements. Another common practice has been to provide separate compliant stiffener elements or springs beneath the foils to supply the required compliance. Examples of typical foil thrust bearings are shown in U.S. Pat. Nos. 5,547,286; 4,871,267; 4,682,900; 4,668,106; 4,624,583; 4,621,930; 4,597,677; 4,459,047; 4,331,365; 4,315,359; 4,300,806; 4,277,113; 4,277,111; and 4,247,155.
Notwithstanding the inclusion of the above design characteristics, the load capacity of a foil thrust bearing still depends on the compliance of the bearing with pressure exerted by a fluid film developed between the bearing and the runner. The pressure profile for a thrust bearing varies, and in order to accommodate the optimal pressure profile and attendant fluid film thickness associated with maximum load capacity, the thrust bearing should be designed to provide stiffness that varies in a manner similar to the pressure profile.
Current foil thrust bearings have limited load capacity. This limitation results from spring designs that indicate only a limited appreciation for variance in pressure profile and its effect on load capacity. Some spring designs have been directed towards providing varying stiffness in radial directions. Examples of designs for varying radial stiffness are shown in U.S. Pat. Nos. 5,110,220; 4,668,106 and 4,277,112. However, they tend to provide limited load capacity because of excess pad deflection over the spring support points.
In an effort to address the above limitations in spring design, U.S. Pat. No. 5,248,205 provides a plurality of sets of arcuate springs having a corrugated configuration. Rectangular slots are provided in the individual springs, with the number, size and position of the slots being altered. Such alteration is intended to change both the radial and circumferential stiffness of the springs. However, the need for slots with such variation makes manufacturing of the springs cumbersome.
Another attempt at varying both the radial and circumferential stiffness of the spring in a foil thrust bearing is found in U.S. Pat. No. 5,318,366. Therein, a plurality of corrugated springs is provided in sets. Within any one set, the springs have increasing widths towards the outer edge of the foil. And each spring within a set has a decreasing width towards the leading edge of the foil. But the changing configurations within each set of springs and within each individual spring makes manufacturing burdensome.
As can be seen, there is a need for an improved foil thrust bearing. In particular, there is a need for a foil thrust bearing that provides varying circumferential and/or radial stiffness. A further need is for an improved foil thrust bearing that allows for cirumferential stiffness that can correlate to a varying fluid film pressure developed by the bearing. Another need is for a foil thrust bearing that includes an underspring element that can be manufactured by fewer manufacturing steps when compared to the current technology but still provide varying stiffness.